Conventionally, electric power steering apparatus (EPS) having a motor as a drive source are known. For EPS, various technologies are proposed which utilize a high controllability to realize an excellent steering feeling.
For example, an EPS disclosed in Patent Document 1 optimizes the characteristic of compensation control of each kind based on an assist gradient that is a ratio of a change in an assist force relative to a change in steering torque. That is, the EPS calculates a torque derivative control level that is a compensation component based on the derivative of steering torque. Next, the EPS adds the compensation component to the basic component of an assist instruction value corresponding to a target assist force. Hence, the responsiveness of applying an assist force relative to a change in the steering torque can be enhanced. Accordingly, a response delay of applying an assist force can be suppressed, and thus, for example, a dragging feeling occurring at the beginning of the turning of a steering wheel and a rolling feeling occurring at the end of the turning of the steering wheel can be avoided. Moreover, it becomes possible to efficiently suppress reverse input vibration from turning wheels.
Conversely, within a range where an assist gradient is large, an assist force largely changes relative to a slight change in steering torque. Within such a range, a torque derivative control level, which has a phase advancing characteristic, becomes excessive. Accordingly, the response feeling when turning the steering wheel decreases, and thus the steering feeling is likely to decrease.
According to the above-explained conventional technology, however, by reducing the torque derivative control level in accordance with an increase of the assist gradient, the torque derivative control level can be optimized across a broad control range. Hence, it becomes possible to realize a good steering feeling which has an excellent steering characteristic and has little adverse effect of reverse input vibration.
Moreover, an EPS disclosed in Patent Document 2 includes a first normative model that defines a target steering torque based on a steering angle, and a second normative model that defines a target steering angle (target rolling angle) of a steering system based on steering torque. The actuation of the motor is controlled based on both of those normative models (ideal models). That is, a steering torque is always set to be an optimized value by a first assist component obtained by executing a torque feedback control in order to cause an actual steering torque to follow a target steering torque. Moreover, reverse input vibration from turning wheels can be canceled by a second assist component obtained by performing a steering angle feedback control that causes an actual steering angle to follow a target steering angle. That is, when the actuation of the motor is controlled based on an addition value of the first and second assist components, a steering feeling, which has an excellent steering characteristic and has little adverse effect of reverse input vibration, can be realized.
In recent years, a further high level quietness is required for vehicles. According to the conventional technology disclosed in patent Document 1, however, it is difficult to accomplish both suppression of reverse input vibration and excellent steering characteristic. More specifically, a reduction of small reverse input vibration, i.e., vibration components with a relatively high frequency caused at the time of, for example, straight running and braking is required. In order to satisfy this requisition, when a torque derivative control level is increased, the steering characteristic becomes poor.
Conversely, when the conventional technology disclosed in Patent Document 2 is applied, theoretically, all reverse input vibrations can be suppressed. When, however, the possibility that the two models interfere with each other is taken into consideration, it is difficult to cause an actual steering condition to match both of the normative models. Hence, it is difficult to accomplish both suppression of reverse input vibration and excellent steering characteristic at a high level.